This invention relates to a bubble tube assembly used to determine the level and density of fluids held within a tank or other vessel. More particularly, this invention relates to a method and apparatus for a maintaining a constant air flow rate in bubble tubes that are used to measure the hydrostatic pressure exerted by fluids.
A bubble tube assembly is a device used to measure the hydrostatic pressure of a fluid. A typical bubble tube assembly according to the prior art is shown in FIG. 1.
Referring to FIG. 1, a bubble tube assembly comprises a vertically mounted bubble tube 2 having an open lower end that is immersed beneath the surface of a fluid. An adjustable regulator 1 is connected to the top of bubble tube 2 for supplying a pressurized flow of air to bubble tube 2. Regulator 1 is adjusted until regulator 1 supplies a flow of pressurized air to the top of bubble tube 2 that is sufficient to cause air bubbles to begin to escape from the open lower end of bubble tube 2. A pressure measurement device 3, such as a pressure transducer, meter, manometer or the like, is connected to tube 2 to measure the air pressure within tube 2. The pressure at which air bubbles begin to escape from the open lower end of bubble tube 2 is observed, reported and/or recorded since this air pressure is equal to the hydrostatic pressure exerted by the fluid at the lower end of bubble tube 2.
Once the hydrostatic pressure exerted by the fluid at the lower end of bubble tube 2 is known by virtue of the pressure reading from pressure measuring device 3, this pressure information can be used to easily calculate the depth of immersion of the open lower end of bubble tube 2 within the fluid, provided the density of the fluid is known, as well as the level of the fluid within a tank or other vessel. Moreover, bubble tubes 2 can be used in pairs with one bubble tube 2 being longer than the other and with the distance between the open lower ends of the pair of bubble tubes 2 being known. The pressure differential between the pressures in the pair of bubble tubes 2 can then be used to calculate the density of the fluid. U.S. Pat. Nos. 4,393,705 and 4,694,692, both of which are hereby incorporated by reference, disclose the latter described use of a pair of bubble tubes within a fluid to determine both the level of a fluid and the density or specific gravity of the fluid.
Bubble tube assemblies have been proposed for determining the level and density of various fluids where the values for these parameters may vary significantly and where accuracy is important. Such applications include the fluid levels and densities of sulfuric acid in a flooded lead-acid battery as used in telecommunications of the fluid levels and densities of drilling mud used in the oil and gas well drilling industries. For example, U.S. Pat. No. 4,694,692 discloses use with respect to drilling mud.
Unfortunately, the levels and densities of these types of fluids may fluctuate widely, which necessitates frequent manual adjustments to the airflow to the bubble tubes. For example, the hydrostatic pressure exerted by one inch of fluid is much less than that exerted by eight feet of fluid, and the pressure needed to measure a fluid having a density of 20 pounds per gallon is much greater than that needed to measure a fluid having a density of 8 pounds per gallon. Thus, unless carefully watched and manually adjusted, the air pressures applied to the bubble tubes may be either too high or too low at various times, thus preventing one from obtaining consistently accurate readings. Since the oil well drilling industry also requires accuracy to within one tenth of an inch of water of hydrostatic head, bubble tube assemblies have not generally been considered accurate enough for determining the level and density of drilling mud.
This invention relates to a bubble tube assembly incorporating a diaphragm type pressure regulator that maintains a constant rate of air flow by controlling the output air pressure from the regulator, i.e. the input air pressure to the bubble tube, by using a feedback loop from the bubble tube to the regulator and by conducting the output air pressure from the pressure regulator through a precision, flow limiting orifice before the output air pressure is introduced into the bubble tube.
One aspect of this invention relates to a self-regulating bubble tube assembly that comprises a bubble tube having an open lower end immersed beneath a surface of a fluid. A means is also provided for supplying a constant rate of air flow to the bubble tube despite changes in back pressure within the bubble tube.
Another aspect of this invention relates to a self-regulating bubble tube assembly that comprises a bubble tube having an open lower end immersed beneath a surface of a fluid. A pressure regulator is connected to the bubble tube through a flow limiting orifice for providing a flow of pressurized air to the bubble tube. The pressure regulator is adjustable to provide a variable output air pressure. A feedback loop connects the bubble tube and the pressure regulator to adjust the pressure regulator to change the variable output air pressure from the pressure regulator with changes in back pressure within the bubble tube. An increase in the back pressure within the bubble tube causes an increase in the output air pressure from the pressure regulator and a decrease in the back pressure within the bubble tube causes a decrease in the output air pressure from the pressure regulator.
Yet another aspect of this invention relates to a method of using a bubble tube assembly. The method comprises inserting a bubble tube having an open lower end into a fluid such that the open lower end of the bubble tube is immersed beneath a surface of the fluid, applying a flow of pressurized air to the bubble tube, and maintaining the rate of the air flow to the bubble tube constant despite changes in back pressure within the bubble tube.